The present invention relates to a method and a device for locating a workpiece with respect to a tool.
As is already known, for processing metal section members or sheet metal and plate elements of any desired thickness and size, numerical control apparatus are conventionally used both for locating and for properly machining the workpiece, by drilling, milling, punching, cutting and other machining operations performed by disc or strip plates, or by thermal workpiece processing sources.
In particular, for carrying out the above mentioned machining operations, a prior machining method provides to grip the section member by one or more grippers supported by a gripper carriage sliding on sliding guides and controllably driven by a servomotor and geared unit assembly.
In this method, data related to the position, speed and driving direction of the workpiece are derived from an encoder or a resolver which are operatively coupled to the section member driving carriage.
Thus, this method provides parameters related to the carriage being operatively driven, but not to the actual position of the workpiece being machined with respect to its machining tools.
Thus, while the above mentioned prior system provides a comparatively high machining precision, it has the drawback of a comparatively large size and length, since the carriage driving means have a length at least corresponding to the workpiece length.
In addition to the above drawback, one must also consider the length of the workpiece and grippers and gripper control means, which length at least corresponds to that of the workpiece processing units.
In yet another prior workpiece processing system the workpieces are driven by driving rollers at least one of which is driven by a motor causing in turn a workpiece to be driven. While this latter prior system has a size less than that of the gripper carriage system, it, however, has a poor processing precision, in particular as the workpieces being machined have a large length.
Such a poor machining precision being due to the arrangement of encoder of resolver devices on the rotary axis of the driving roller or of the measuring roller.
In fact, both the above prior systems must necessarily transform a rotary movement to a linear movement by using, for this transforming operation, the value 3, 14 . . . corresponding to π which, as is known, is not an entire number and, accordingly, in a rotary assembly, generates a workpiece positional error.
A further drawback of the above latter system is that the driving and measuring rollers are subjected to a comparatively high wear undesirably changing the roller circumferences, thereby the workpiece cannot be precisely located with respect to its machining tool, since the roller wear, usually generated by small section member workpiece, is uneven on the overall roller circumference and height of the rollers.
Finally, it should be also added to the above that said rollers frequently have, to provide a sure gripping of the roller and section members, a knurled texture, thereby said rollers operate on variable diameters, which change depending on the variable pressure pressing said rollers against the section member being machined.